defined() not recognizing ## token paste in its operand (GNU extension)

[wjakobsson]

When defined() appears inside a macro with ## op, syntaxError gets emitted.

error: failed to evaluate #if condition, undefined function-like macro invocation: defined( ... ) [syntaxError]
#if NRF_MODULE_ENABLED(FOO)

Reproducer: main.c

#define NRF_MODULE_ENABLED(module) \
    (defined(module ## _ENABLED) && (module ## _ENABLED))

#define FOO_ENABLED 1

#if NRF_MODULE_ENABLED(FOO)
int foo(void) { return 1; }
#endif

This is caused by the DEFINED branch in expandToken, which doesn't match defined(A ## B), and makes the token 'defined(...)' fall through.

[firewave]

Looks related to #300, #218 and #75.

gcc -E and clang -E show:

int foo(void) { return 1; }